Production of low nitrogen high chromium ferrous alloys

ABSTRACT

A low-nitrogen low-carbon high-chromium steel is made by forming a melt of chrome ore, iron ore and lime, reducing the melt with silicon or ferrosilicon out of contact with the ambient air, and teeming the underlying metallic phase also out of contact with the ambient air into a mold.

United States Patent [1 1 Aukrust et al.

. Aug. 28, 1973 1 PRODUCTION OF LOW-NITROGEN HIGH-CHROM'IUM FERROUS ALLOYS [75] Inventors: Egil Aukrust, Bethel Park; T. Grant John, Pittsburgh, both of Pa.

[73] Assignee: Jones & Laughlin Steel Corporation,

Pittsburgh, Pa.

[22] Filed: Apr. 16, 1971 [21] Appl. No.: 134,831

[52] US. Cl 75/l30.5, 75/51, 75/133.5 [51] Int. Cl. C22c 33/00 [58] Field of Search 75/130.5, 60, 133.5,

[56] References Cited UNITED STATES PATENTS 11/1937 Udy 75/11 7/1940 Udy 75/130.5

' Primary Examiner-L. Dewayne Rutledge Attorney-G. R. Harris and T. A. Zalenski 5 7] ABSTRACT A low-nitrogen low-carbon high-chromium steel is made by forming a melt of chrome ore, iron ore and lime, reducing the melt with silicon or ferrosilicon out of contact with the ambient air, and teeming the underlying metallic phase also out of contact with the ambient air into a mold.

8 Claims, No Drawings I PRODUCTION OF LOW-NITROGEN HIGH-CHROMIUM FERROUS ALLOYS This invention relates to the production of lownitrogen high-chromium alloys and more particularly to methods for producing low-nitrogen high-chromium steels. By high-chromium steels, we mean steels having chromium contents such as those of the 400 series and higher. Our invention to be described includes methods for making such steels with or without additional alloy ing elements such as nickel, molybdenum, copper, etc,, and also for making low-nitrogen ferroalloys of chromium contents high enough to be classified as ferrochromium. These contain upwards of 40 percent chromium.

High-chromium steels find many applications in the manufacture of articles which must resist corrosion. It has been found that the corrosion resistance of such steels in many corrosive mediums is improved if their carbon content is reduced to a low level, less than 0.03 percent, and these extra-low-carbon corrosion resistant steels, so-called, command premium prices. It has also been found that the low carbon content of these steels brings about improvement in other properties, such as weldability and ductility. To produce steels of the desired low carbon content some investigators in recent years have turned away from conventional steelmaking processes, such as basic oxygen furnace, electric furnace, and the like, which utilize starting materials of relatively high carbon content and burn most of it out during refining. These conventional processes have the disadvantage that they oxidize chromium in preference to carbon at ordinary furnace temperatures and even above temperatures of 3,200F or so. Thus they are not economic for the production of extra low carbon steels. Recently, therefore, attention has been directed to processes similar to those long employed to make ferroalloys-the reduction of lime-oxide melts. The starting material in these processes is almost carbon-free.

It has been found that the corrosion resistance and other physical properties of the extra-low-carbon highchromium steels can be still further improved if their nitrogen contents are lowered. The only way of making steels of such compositions known prior to our invention to be described was by vacuum melting or refining of carefully selected starting materials. While vacuum refining will reduce the content in steel of certain elements that gasify at low pressure or can be converted into gaseous compounds, for example, carbon, it is not very effective in reducing the nitrogen content of chromium-containing alloys. One way to obtain steel of the desired composition is to melt under vacuum a charge free from chromium, so as to reduce its carbon and nitrogen contents to the desired value, and then to obtain the desired chromium content by adding electrolytically produced chromium, which is substantially nitrogen-free. However, this metal costs more than a dollar a pound.

It is an object of our invention to provide a process of making low-nitrogen as well as low-carbon highchromium steels which does not require vacuum melt ing or refining. It is another object to provide such a process which does not require expensive starting materials. It is another object to provide such a process which is adapted to the production of higher chromium steels than are now conventionally employed, and of ferrochromium. It is still another object to provide such a process which is adapted to produce a highchromium steel and a ferrochromium alloy from the same starting melt. Other objects of our invention will appear in the course of the description thereof which follows.

I The starting material for our process is a melt of chromium oxides, and lime, to which iron oxides are added as required by the composition of the desired product. For steelmaking, we prefer to utilize chromium ore, iron ore and burnt lime. A melt of these constituents containsabout 0.01 to 0.015 percent carbon, and is virtually free of nitrogen or nitrogen compounds which can be transferred to the steel during silicon reduction. The proportions of chromium ore and iron ore are adjusted to provide the required-contents of chromium and iron in the steel produced. The lime addition is proportioned to flux the gangue of the ores, as well as the silicon content of the reducing agent to be described. We melt the constituents in an electric furnace or other suitable melting furnace and bring the melt to a temperature of about 3,000 to 3,300F. Although chromium-containing steels made in arc furnaces usually contain substantial amounts of nitrogen, ranging from 0.035 to 0.06 percent, which is believed to come from dissociation of air by the arc, we find that the nitrogen content of our lime-oxide melts is only a few parts per million. g

The lime-oxide melt above described is then poured into a ladle or other vessel for the reduction step. The ladle is open to the ambient air and the pouring is conducted in th ambient air. In this ladle, the lime-oxide melt is reacted with the reducing agent, which is introduced on top of the melt. The reducing agent is silicon or ferrosilicon, or a mixture of both. The quantity of reducing agent is adjusted to contain sufficient silicon to reduce most of the chromium and most of the iron contained in the oxide-lime melt to metallic form. The efficiency of the reducing agent with respect to these elements depends, of course, on the composition of the reducing agent, its physical form and other factors familiar to those skilled in the art, and must be determined by experiment. We add a quantity of reducing agent calculated to leave about 2 percent of the iron and from about 2 to 6 percent of the chromium in the slag. If more reducing agent is added, there is a risk of obtaining an excessively high silicon content in the steel. The reducing agent is added gradually over an appreciable period of time to the lime-oxide melt in a ladle open to the air and the slag and metal phases are sampled and analyzed from time to time. The addition of the reducing agent is controlled in accordance with those analyses. The reducing agent must be low in nitrogen and, if a low carbon content is also desired in the steel, as is usually the case when low nitrogen is desired, the reducing agent must be low in carbon as well. Ferrosilicon containing about 40 percent or more .silicon, not more than about 0.02 percent carbon and not more than about 0.004 percent nitrogen, is commercially available and is satisfactory for our process. Ferrosilicon containing 0.005 to 0.010 percent carbon and not more than 0.0025 percent nitrogen can be obtained.

The reducing agent is added to the lime-oxide melt preferably as a solid, or is melted and added as a liquid. We prefer solid material crushed to less than 1 inch but greater than 8 mesh. In either case, the addition takes place in the ambient air. The reducing agent reduces chromium and iron from the lime-oxide melt within the melt volume into metallic form and the molten metallic alloy so formed sinks to the bottom of the ladle. The slag formed by the silicon with the constituents of the lime, CaO, A1 0,, and MgO, floats on top of the alloy phase and prevents contact of the ambient air therewith. The relative proportions of lime, oxides, and reducing agent are preferably adjusted so that the basicity ratio of the slag, after completion of the reduction, is between about 1.0-2.0, although we have successfully produced product with basicities ranging from 0.5 to 4.0. This ratio is calculated by dividing the sum of the percentages of CaO and MgO in the slag by the percentage of SiO The rate of addition of the reducing agent is controlled so as to avoid splashing which would expose the reduced alloy to the ambient air.

The reaction of the reducing agent with the limeoxide melt generates heat and agitates the resulting bath, so promoting thereducin'g action, but the mixing.

so effected is preferably supplemented to carry the reaction to completion in a reasonable period of time. As

we have mentioned, it is more desirable to reduce as much as possible of the chromium into the metallic phase, and the latter must contain no more silicon than is permitted by the specification for the steel to be produced. The mixing action should be sufficient to bring about reaction between the reducing agent and melt before the reducing agent sinks to the bottom of the ladle, otherwise an undue proportion of the silicon is found in the metallic phase. Furthermore, the mixing must not expose the metallic phase to the ambient air, as we have found that molten chromium-iron alloys have a pronounced tendency to adsorb nitrogen from surround air. We prefer to facilitate the mixing of the reducing agent and lime-oxide melt by bubbling a nonreactive gas through it. We find that agron is suitable for this purpose, although any of the other rare gases neon, xenon, krypton, or helium can be used. We prefer to introduce the nonreactive gas through a porous plug in the bottom or lower side wall of the ladle, but it can also be introduced through tuyeres in those locations or through lances or tuyeres immersed in the bath from above.

When the reducing reaction has proceeded to the desired point, gas bubbling is discontinued and the alloy phase is teemed from the ladle through a nozzle in its bottom into a mold, or tapped into a holding ladle if further additions are necessary. This teeming and tapping must be done so as to keep the molten alloy out of contact with the ambient air. It is conveniently carried out in an evacuated chamber which encloses the nozzle and the mouth of the mold or holding ladle. It is also satisfactory to teem the steel within a shroud of a nonreactive gas which envelops the stream-of metal and the mouth of the mold. The steelis cast into individual ingot molds under reduced pressure, or atmospheric pressure, or is continuously cast into an open bottom continuous casting mold.

EXAMPLE I It was desired to produce a steel containing about 26 percent chromium, about 1 percent molybdenum, not more than 1 percent silicon, carbon and nitrogen as low as possible, with the balance being iron. The lime-oxide melt was made from South African chromium ore, Hi1- ton iron ore pellets and lime. The chromium ore contained 31.0 percent Cr, 19.0 Fe, 1.7 percent SiO 10.9 percent MgO, 0.03 percent CaO and 15.3 percent Al- 0 the iron ore contained 65.0 percent Fe, 2.0 percent SiO,, 1.1 percent A1 0 0.06 percent CaO and 0.02 percent MgO; the lime analyzed 94.5 percent .CaO, 2.5 percent MgO, 0.06 percent SiO, and 0.5 percent Al O The melt, comprising 18,400 pounds of chromium ore, 6,380 pounds of iron ore and 14,250 pounds of lime, was brought to a temperature of about 3,000 in an arc furnace and was then tapped into a reaction ladle. lnto the melt in the ladle was introduced 8,600 pounds of solid crushed ferrosilicon containing 49 percent Si, 0.007 percent C and 0.002 percent N. The resulting bath was stirred by bubbling argon through it at a rate of about 20 standard cubic feet per minute so as to facilitate the reducing reaction, and the bubbling was continued until the silicon content of the underlying alloy phase fell below 1 percent. The total reaction time was about 39 minutes. A coolant comprising 1,800 pounds of slag corresponding to the slag below mentioned was added during the reaction, together with 200 pounds of molybdenum as molybdenum oxide. The metal phase was then teemed into an ingot mold through an argon shroud, and amounted to 15,500 pounds of steel having an analysis of 24.3 percent Cr, 0.73 percent Si, 0.008 percent C, 0.004 percent N, 1.41 percent Mo, 0.010 percent S, 0.030 percent P, with the balance being iron. The slag discard, which amounted about 43,500 pounds, contained 32.7 percent CaO, 8.2 percent A1 0 20.9 percent SiO 29.0 prcent MgO, 4.6 percent Cr and 1.6 percent Fe.

It will be observed that the chromium-containing ore used in the lime-oxidemelt of Example I was a relatively cheap ore having a chromium-to-iron content ratio of about 1.5. Ores of this type are not satisfactory for the production of metallurgical grade low carbon ferrochromium, which requires an ore charge with a chromium-to-iron ratio of 3:1 or greater. Our process above described utilizing chromium ores of the type above described is adapted to be carried out in two states, the first stage producing a low-carbon, lownitrogen high-chromium steel, such as the type above mentioned, and the second a low-carbon metallurgical grade ferrochromium. This result is possible because the reducing agent of our process reduces iron from the lime-oxide melt in preference to chromium. When our process is carried out in this way, the lime-oxide melt is formed from lime and chromium ore alone. The quantity of reducing agent added in the first stage is less than that required to reduce all the iron and chromium from the melt. Most of the iron, but only a portion of the chromium, then appears in the alloy phase. This phase is tapped out as has been described, and the melt remaining is again treated by an addition comprising the balance of the reducing agent and additional chromium, if required. This second addition reduces the remaining iron and chromium of the melt so that the alloy phase is .ferrochromium of the desired chromium content.

EXAMPLE 11 g It was desired to produce a steel of the composition of Example 1, except for molybdenum, together with metallurgical grade ferrochromium from a Transvaal chromium ore analyzing 45.24 percent cr,o,, 24.3 percent FeO, 1.57 percent SiO 14.94 percent A1 0 and 11.76 percent MgO. 29,500 pounds of this ore, together with 18,000 pounds of lime of the same grade as that of Example 1, were melted together in an arc furnace and brought to a temperature between 3,000 and 3,200F. The melt, having a composition of 19.3 per.- cent Cr, 11.8 percent Fe, 1 percent SiO,, 9.3 percent M 0 7.3 percent MgO and 35.5 percent CaO, was poured into a reaction ladle to which was added 5,100 pounds of 50 percent ferrosilicon. The resulting bath was stirred by bubbling argon through it in the manner which has been described. A metal phase comprising about 10,000 pounds of steel, analyzing 26.5 percent Cr, 1 percent Si, 0.01 percent C and 0.002 percent N was teemed into an ingot mold in an evacuated chamber as has been described. To the melt remaining in the ladle was added 3,800 pounds of 50 percent ferrosilicon and 1,500 pounds of FeCrSi; reladling was utilized to facilitate the reaction. An alloy phase of 8,700 pounds of low carbon ferrochromium analyzing about 70 percent chromium was tapped therefrom. The remaining slag, which was discarded, contained 1 percent Cr, 0.5 percent Fe, 29.6 percent SiO,, 11.1 percent Al- 0 9.8 percent MgO and 42.4 percent CaO.

High chromium steels containing alloying elements other than molybdenum are made in the same way as the steel of Example I. As our process is a reducing process, it permits the alloying constituent to be added as an oxide, for exaple, 18-8 stainless steel is made by adjusting the melt composition to produce the required chromium content in the alloy phase and adding nickel to the melt in the form of nickel-oxide during or after the reducing reaction.

We claim:

1. The process of making an iron-chromium alloy containing only a few parts per million nitrogen comprising forming a melt of substantially nitrogen-free chromiun oxides, iron oxides and lime, bringing into contact therewith a low-nitrogen silicon-containing reducing agent, mixing the melt and reducing agent to form a reduced alloy phase beneath a slag phase without exposing the alloy phase to air, and teeming the alloy phase into a mold without exposing it to air.

2. The process of claim 1 in which the melt and reducing agent are mixed by bubbling therethrough a gas which is nonreactive therewith.

3. The process of claim 2 in which the reducing agent is added to the top of the melt and the, rate of flowof gas bubbled therethrough is adjsuted to bring about complete reaction between melt and reducing agent before the reducing agent sinks to the bottom.

4. The process of claim 2 in which the melt and reducing agent are mixed in a ladle open to the ambient air and the bubbling of the gas is controlled to avoid exposing the reduced alloy phase to air.

5. The process of claim 1 in which the melt and reducing agent are mixed by adding the reducing agent to the melt in a ladle open to the ambient air and the rate of adding the reducing agent to the melt is controlled to avoid exposing the reduced alloy phase to air.

6. The process of claim 1 in which mixing of the melt and reducing agent is continued until the silicon content of the reduced alloy phase is brought down to the value desired in the alloy phase.

7. The process of claim 1 in which the amount of reducing agent added is less than that required for complete reduction of the melt and is proportioned to adjust the constituents ofthe reduced alloy phase to a desired value by preferential reduction of iron over chromium.

8. The process of claim 7 in which the slag phase remaining after the reduced alloy phase has been tapped is mixed with the remaining proportion of reducing agent to form a second reduced alloy phase higher in chromium than the reduced alloy phase. 

2. The process of claim 1 in which the melt and reducing agent are mixed by bubbling therethrough a gas which is nonreactive therewith.
 2. The process of claim 2 in which the reducing agent is added to the top of the melt and the rate of flow of gas bubbled therethrough is adjsuted to bring about complete reaction between melt and reducing agent before the reducing agent sinks to the bottom.
 4. The process of claim 2 in which the melt and reducing agent are mixed in a ladle open to the ambient air and the bubbling of the gas is controlled to avoid exposing the reduced alloy phase to air.
 5. The process of claim 1 in which the melt and reducing agent are mixed by adding the reducing agent to the melt in a ladle open to the ambient air and the rate of adding the reducing agent to the melt is controlled to avoid exposing the reduced alloy phase to air.
 6. The process of claim 1 in which mixing of the melt and reducing agent is continued until the silicon content of the reduced alloy phase is brought down to the value desired in the alloy phase.
 7. The process of claim 1 in which the amount of reducing agent added is less than that required for complete reduction of the melt and is proportioned to adjust the constituents of the reduced alloy phase to a desired value by preferential reduction of iron over chromium.
 8. The process of claim 7 in which the slag phase remaining after the reduced alloy phase has been tapped is mixed with the remaining proportion of reducing agent to form a second reduced alloy phase higher in chromium than the reduced alloy phase. 